An SOI (Silicon On Insulator) substrate comprises a structure in which a semiconductor substrate, an insulator film, and a semiconductor layer are stacked. A lateral semiconductor device comprising a pair of main electrodes on a surface of the semiconductor layer of the SOI substrate is known. A lateral semiconductor device utilizing an SOI substrate will be characterized in that faulty operation caused by surge voltage does not readily occur, and is expected to be a promising semiconductor device.
FIG. 14 schematically shows a cross-sectional view of essential parts of a lateral diode 300. The diode 300 comprises a semiconductor substrate 320 including a high concentration of p-type impurity, an insulator film 330 formed on the semiconductor substrate 320, and a semiconductor layer 340 formed on the insulator film 330. The semiconductor layer 340 comprises a cathode semiconductor region 352 including a high concentration of n-type impurity, an anode semiconductor region 355 including p-type impurity, and a semiconductor active region 353 including a low concentration of n-type impurity. The semiconductor active region 353 isolates the cathode semiconductor region 352 and the anode semiconductor region 355. The cathode semiconductor region 352 is electrically connected to a cathode electrode. The anode semiconductor region 355 is electrically connected to an anode electrode. The semiconductor substrate 320 is fixed at the same potential as the anode electrode.
When a voltage higher than the anode semiconductor region 355 is applied to the cathode semiconductor region 352, the diode 300 assumes a non conducting state. At this juncture, a depleted layer 351 extends within the semiconductor active region 353 (the broken line shows an edge face of the depleted layer) from a pn boundary surface between the anode semiconductor region 355 and the semiconductor active region 353. Since the semiconductor substrate 320 is fixed at the same potential as the anode electrode, field plate effects are exerted on the depleted layer 351. As a result, the depleted layer 351 extends along the insulator film 330. As a result, a wide range of the semiconductor active region 353 is depleted, including a portion below the cathode semiconductor region 352. The semiconductor active region 353 can thus bear the potential difference between the cathode region 352 and the anode region 355. The withstand voltage of the diode 300 is restricted to the lower voltage out of: the voltage borne by the electrical field formed in a lateral direction between the cathode semiconductor region 352 and the anode semiconductor region 355, and the voltage borne by the electrical field formed in a vertical direction between the cathode semiconductor region 352 and the semiconductor substrate 320. The voltage borne in the lateral direction can be increased by lengthening the width in the lateral direction of the semiconductor active region 353. As a result, it is necessary to increase the voltage borne by the electrical field formed in the vertical direction between the cathode semiconductor region 352 and the semiconductor substrate 320 in order to increase the withstand voltage of the diode 300.
It is desirable to increase the voltage borne by the insulator film 330 so as to increase the voltage borne in the vertical direction. The thickness of the insulator film 330 may be increased in order to increase the voltage borne by this insulator film 330. However, increasing the thickness of the insulator film 330 creates the problem of increasing the time needed to form the insulator film 330. Furthermore, there is also the problem that, when the thickness of the insulator film 330 is increased, the depleted layer 351 caused by the field plate effects extends for a shorter distance. It is consequently not expedient to increase the thickness of the insulator film 330. A technique is thus desired for increasing the voltage borne in the vertical direction between the cathode semiconductor region 352 and the semiconductor substrate 320 while keeping the thickness of the insulator film 330 within a predetermined range.
For this purpose, it is desirable to increase the voltage (or the electrical field) that can be borne by the insulator film 330 per unit thickness. It is known that the voltage that can be borne by the insulator film 330 per unit thickness is usually approximately three times the critical electrical field at the boundary surface between the semiconductor active region 353 and the insulator film 330. As a result, an effective measure for increasing the voltage that can be borne by the insulator film 330 per unit thickness is to increase the critical electrical field at the boundary surface between the semiconductor active region 353 and the insulator film 330.
In T. Letavic, E. Arnold, M. Simpson, R. Aquino, H. Bhimnathwala, R. Egloff, A, Emmerik, S. Mukherjee, “High Performance 600V Smart Power Technology Based on Thin Layer Silicon-on-Insulator”, ISPSD, 1997, p. 49-52 a semiconductor device is proposed in which the thickness of a semiconductor active region has been greatly reduced. The thickness of the semiconductor active region is adjusted by means of a field oxidizing layer formed on a surface of the semiconductor active region. That is, the thickness of the semiconductor active region is adjusted by adjusting the depth that the field oxidizing layer extends into the semiconductor active region. The thickness of the semiconductor active region is adjusted so as to be thin when the field oxidizing layer extends to a deep position within the semiconductor active region. When the semiconductor active region is thin, the distance is reduced that carriers must move in the vertical direction along an electrical field that is formed in the vertical direction. The avalanche breakdown occurs when the value of the ionizing rate of the carriers integrated with the distance of movement, i.e. the ionization integral, reaches 1. When the semiconductor active region is thin, there is a reduction in the distance that carriers must move, and the occurrence of the avalanche breakdown can be controlled. For this reason, in the semiconductor device of T Letavic, et al, it is possible to control the occurrence of the avalanche breakdown even though the electrical field at the boundary surface between the semiconductor active region and the insulator film has been increased. In the semiconductor device of T. Letavic, et al, therefore, the critical electrical field at the boundary surface between the semiconductor active region and the insulator film can be increased, the voltage that can be borne by the insulator film per unit thickness can be increased, and the voltage that can be borne by the insulator film can be increased.